In general, a signaling process for wireless communication at a transmission side includes a channel coding process, a modulation process, a frequency up-converting process, and a transmission process. In response to this, a signaling process at a reception side includes a frequency down-converting process, a demodulation process, and a channel decoding process. Herein, the demodulation process at the reception side includes a process of calculating a decoding metric per bit or symbol. In general, a Log Likelihood Ratio (LLR) is broadly used as the decoding metric.
In order to generate the LLR, it is needed to premise a specific probability distribution for interference and noise signals. A conventional technology assumes a Gaussian distribution for interference signals to perform decoding at a low complexity. Accordingly, in order to make characteristics of the interference signals to be maximally similar to the Gaussian distribution, modulation schemes of Quadrature Amplitude Modulation (QAM) series are mainly used. However, it is generally known that channel capacity of a non-Gaussian channel is greater than that of a Gaussian channel. Accordingly, if suitable decoding is performed, decoding performance may be obtained in the non-Gaussian channel to be higher than that of the Gaussian channel.
Therefore, it will be needed to develop a modulation scheme for following inference signals to a non-Gaussian distribution. As a result, a Frequency QAM (FQAM) scheme is suggested. The FQAM scheme as a hybrid modulation scheme in which a QAM scheme and a Frequency Shift Keying (FSK) modulation scheme are combined has all advantages in that spectrum efficiency is high in the QAM scheme and interference signals of the FSK modulation scheme are converted into non-Gaussian signals.
Because the FQAM scheme is a modulation technology which is newly suggested, there is no method of calculating an LLR which is a decoding metric. Because one symbol occupies a plurality of tones on a frequency axis in the FQAM scheme to differ from the QAM scheme, it is unsuitable to apply a method of calculating an LLR in the QAM scheme to the FQAM scheme. That is, if the method of calculating the LLR in the QAM scheme is applied to decode a symbol generated by the FQAM scheme, there may be great performance deterioration. Therefore, it is needed to provide an effective method of calculating an LLR in the FQAM scheme.